1. Field of the Invention
The present invention relates in general to hydraulic brake systems of wheeled motor vehicles, and more particularly to the hydraulic brake systems of a type having both an antilock brake system (viz., ABS) and a vehicle motion stabilizing control system which will be referred to as "VMSCS" hereinafter for ease of description. As is known, the ABS is a system for suppressing a lock of road wheels upon braking. In the VMSCS, there are two types, one type being a traction control system which functions to suppress a slip of drive road wheel or wheels by applying a controlled braking force to the drive road wheel or wheels in accordance with the running condition of the vehicle, and the other type being a yaw rate control system which, for stabilizing the attitude of the vehicle, controls the yaw rate of the vehicle by applying a controlled braking force to a selected road wheel or wheels in accordance with the running condition of the vehicle.
2. Description of the Prior Art
Japanese Patent First Provisional Publication 7-50106 shows a hydraulic brake system of the type having both the ABS and the VMSCS. In the brake system of this publication, an ABS unit is installed in a hydraulic brake circuit which extends from a mastery cylinder to each of brake cylinders of road wheels. The ABS unit comprises generally a hydraulic pressure control valve which, for controlling a hydraulic pressure of the brake cylinders, includes inlet and outlet valves, a reservoir which temporarily reserves the brake fluid discharged from the hydraulic pressure control valve, and a main pump which sucks the brake fluid from the reservoir and returns the same to the brake circuit. Through a normally closed electromagnetic valve (viz., inside gate valve) which opens only when the VMSCS operates, the main pump sucks a pressurized fluid from an auxiliary pump and sucks the brake fluid from the master cylinder.
Accordingly, under operation of the VMSCS, the main pump can feed the brake circuit with a pressurized brake fluid even when a brake pedal is not actuated. That is, due to the pressurized fluid thus fed from the main pump, the ABS unit can increase the pressure in each brake cylinder and thus produce a braking force.
As the main pump, a so-called plunger type is commonly used wherein a plunger is reciprocatively moved in a cylinder member by a rotating cam. FIG. 4 shows schematically a pump 100 of such plunger type. Denoted by numeral 102 is a cam driven by a motor (not shown), and 104 is a plunger whose base end contacts the cam 102. The plunger 104 is slidably received in a cylinder member 106 in a manner to define a pressure chamber 108 between a leading end of the plunger 104 and a bottom of the cylinder member 106. A seal member 107 is arranged between a cylindrical outer surface of the plunger 104 and a cylindrical inner surface of the cylinder member 106 to achieve a hermetical sealing therebetween. The pressure chamber 108 is communicated with an outlet circuit 110 through an outlet valve 112 installed in the bottom of the cylinder member 106. The outlet circuit 110 is communicated with a brake circuit (not shown). Furthermore, the pressure chamber 108 is communicated, through an inlet valve 114 installed in the plunger 104, with both a first inlet circuit 116 connected to a reservoir 118 and a second inlet circuit 120 connected to a master cylinder. The first and second inlet circuits 116 and 120 include respective openings 124 and 126 formed in a cylindrical wall of the cylinder member 106. Within the second inlet circuit 120, there is installed an inside gate valve 122.
Thus, under operation of the ABS wherein the inside gate valve 122 is in its closed condition, the main pump 100 sucks the brake fluid from the reservoir 118, compresses the same in the pressure chamber 108 and discharges the same to the brake circuit through the outlet circuit 110. While, under operation of the VMSCS wherein the inside gate valve 122 is in its open condition, the main pump 100 sucks the brake fluid from the master cylinder and the auxiliary pump through the second inlet circuit 120 and discharges the same to the brake circuit through the outlet circuit 110.
In the above-mentioned conventional brake system, the inlet valve 114, which is a check valve, works for both the first and second inlet circuits 116 and 120. That is, the inlet valve 114 is installed in the junction part of the two circuits 116 and 120, which part is located in the plunger 104, as shown. In other words, the fluid communication between the pressure chamber 108 and each of the first and second inlet circuits 116 and 120 is made through the opening 124 or 126 which faces the cylindrical outer surface of the plunger 104 throughout the stroke of the plunger 104.
However, due to its inherent construction, the abovementioned brake system has the following drawback which tends to appear when a driver actuates a brake pedal under operation of the VMSCS.
As is mentioned hereinabove, under operation of the VMSCS, the inside gate valve 122 is opened and thus the main pump 100 sucks the brake fluid from, the master cylinder and the auxiliary pump. If, under this operation, the driver depresses the brake pedal to slow down the associated vehicle, the pressure produced in the master cylinder is transmitted between the plunger 104 and the cylinder member 106 through the second inlet circuit 120. Thus, upon the brake pedal depression, a closed clearance defined between the inlet valve 114 and the seal member 107 is highly pressurized, which tends to bring about a function-failure of the seal member 107. For eliminating such failure, a measure has been proposed, in which a master cylinder pressure sensor and/or a brake pedal stroke sensor is employed and the inside gate valve 122 is closed when the pressure in the master cylinder increases beyond a permitted level. However, even this measure has failed to satisfy users because of increased cost of the brake system due to employment of such additional and parts.